In order to provide a greater number of users access to the same communication medium, a variety of multiplexing techniques have been devised. The two principal techniques are known as frequency division multiplexing (FDM) and time division multiplexing (TDM). Applications of FDM and TDM in the telephone industry have made possible the use of two pairs of wires by as many as 24 and more users simultaneously. Attempts to capitalize on the small probability that all users of a TDM system are "off hook" at the same time have led to the use of switching devices called concentrators. These devices, one located in the central office and the other usually located near a remote group of subscribers to be served, allow up to three or more times as many subscribers to have potential access to the central office as the TDM system (typically PCM, pulse code modulation) will actually handle at the same time (usually 24). Their disadvantage is that, in the improbable event that more than, say, 24 subscribers want access to the system, only the first 24 will be served; the others must wait until a free channel is available.
Because the narrow pulses used in TDM systems require a rather wide pass band for their faithful transmission, it is necessary to insert regenerative repeaters at specific locations along a cable to compensate for the high attenuation of the higher frequencies by the cable. Such repeaters generate a new pulse in place of each one detected at the input of the repeater. The detection process is hampered by a phenomenon known in the industry as "pulse jitter" and results in random errors in the pulse stream which show up as noise in the user's receiver.
A combination of FDM and TDM techniques (frequency-time division multiplexing, FTDM) is used in such systems as RADA (Random Access Discrete Address) and is the subject of several patents and articles. Information is communicated in these systems by means of an addressing scheme wherein bursts or pulses of RF energy are transmitted at specified frequencies and at specified moments of time. This addressing scheme can be better appreciated by using the concept of the frequency-time (FT) matrix which is an array of "holes" in the frequency-time domain, each hole being defined by a specified frequency and a specified time slot within a larger interval of time referred to as a time frame. An address consists of a set of two or more RF pulses which occupy predetermined holes in the FT matrix. Because of the large number of addresses which can be defined in this manner on the FT matrix, each bit of user information can be assigned a unique address.
FTDM systems employing this type of addressing are subject to the phenomenon of "false addressing" wherein addresses not intentionally transmitted are, nevertheless, received because RF pulses, produced by a multiplicity of users, randomly occupy hole combinations on the FT matrix which have been defined as user addresses. Such false addresses generate a noise background (self-interference noise) which increases rapidly as the number of users increases.
In improving the FTDM system of the present invention disclosed and claimed in the above-identified patent application, particularly as regards the reduction of self-interference noise, it was determined that the FT matrix has a much greater potential for information content than previously comtemplated. It is to be noted that present FTDM systems are based upon a two-valued or binary condition for each hole in the FT matrix, i.e., each hole has only the presence or absence of a pulse. The present invention takes advantage of this greater potential and additionally eliminates the problem of the self-interference noise. Furthermore, through a radix conversion process in accordance with the present invention, it is possible to concentrate more users into the system with no substantial increase in the required bandwidth and without diminishing the accessibility of users to the system as is the case with existing concentrators. Finally, the typical decision circuitry of TDM system receivers is based upon detection of narrow, noise-like pulses and, in the presence of normal noise, it is difficult for a receiver to distinquish between noise and legitimate pulses; thus, when noise accompanies the desired information, this TDM receiver output is likely to contain digital errors. The present invention significantly improves the decision margin by providing for the reception of several repeated events before the decision is made regarding the correct state. It is also noteworthy that TDM systems operating with telephone cable are susceptible to localized changes in cable characteristics which give rise to different propagation times for different frequencies resulting in pulse shape distortion. In the present invention the use of sinusoidal RF carriers in the FT matrix makes the system relatively immune to problems arising from cable abnormalities.
The greater potential for information content in an FT matrix as provided herein makes the present invention particularly adapted to very high speed data transmission.
Reference is made to the above-identified U.S. Pat. No. 3,872,255 of the present inventors and art cited in the prosecution thereof for an identification of prior art patents and publications in the field of the present invention and attention is also invited to the following: U.S. Pat. No. 3,806,655, U.S. Pat. No. 3,769,461 and Bell System Technical Journal, Vol. 53, No. 5, pages 867 to 936.